1. Field of the Invention
This invention relates to a multiple filament lamp, and more particularly to a PWM-controlled 12V-multi filament automotive lamp supplied by a 42V power net system which has a power net arc detection circuit.
2. Background of Related Art
The tungsten filament of an automobile lamp, for example, is contaminated with iron. This contamination is inherent in the lamp""s manufacturing process. This causes the lamp to be much more likely to arc when it is new. As the lamp is operated it is considered to be seasoning. This seasoning of the lamp refers to the burning off of the iron contamination from the filament. As the lamp becomes more seasoned it is less likely to have arcing faults.
In a multi-filament lamp, when an arc occurs in the lamp it is typically between the filaments. The arc faults are from filament to filament with a voltage appearing on the un-used filament. This voltage varies quite a bit depending on the arc points on the filament. As the filament is operated (seasoned) it is changed from the metal state of tungsten to the crystalline state when cold. The crystalline state is much more brittle consequently making it more fragile as the lamp is used. As the filaments are operated (seasoned) the arcing problem is less likely to occur. It is important to realize that conventional automotive lamps were intended to operate at 16 volts or less. The threshold for arcing (UL statement) is 16V or more.
Since lamps are inherently contaminated with iron during the manufacturing processes, when the lamp is heated for the first few minutes this iron boils off. This conductive iron vapor can and does reach the other filament to form a current path. Typically, both filaments have a common connection and separate power feed lines, as shown in FIG. 1A. When this xe2x80x98shortxe2x80x99 occurs due to iron boil off, energy is transferred from the operating filament to the non-operating filament as well. Then, the operating filament is vaporized and destroyed.
The equivalent circuit, in the event of a short, is shown in FIG. 1B, where FIG. 1B illustrates the operating filament and a portion of the non-operating filament. With more current flowing through a smaller piece of filament [FC] it gets very hot and melts, resulting in failure. The other part of the short is illustrated as two filament portions in parallel relative to the common connections.
The conventional multiple filament lamps have many problems protecting the lamp filaments and do not offer any arc protection for new filaments contaminated with iron.
Accordingly, this invention addresses the arcing problem associated with PWMing 12V multi filament automotive lamps, especially from the 42V power net system, and these same lamps operating on 12V systems. It is an object of the invention to provide a multiple filament lamp arc detection circuit which detects the start of an arc in the multiple filament lamp, especially in higher voltage systems such as power net, and enables the driver circuitry for the lamp to turn off the lamp, before damage occurs to a multiple filament lamp. Such multi-filament lamps include headlamps and any multiple lamps, such as brake lights, taillights, tail lamps. This invention is also applicable whether the lamp is driven from a higher voltage, PWM or not. As an alternative, lamp protection can be provided by reducing the applied voltage to below an arcing level. While arcs are more likely occur in new filaments, even seasoned lamps may undergo arcing and other fault events. The invention is also applicable to prevent lamp failure under these conditions.
The present invention solves the above problems by measuring the voltage rise in the non-operating filament and detecting the occurrence of the arc in the lamp. The measurement then becomes the fault signal which, in turn, causes the drive circuitry to turn off the lamp or attenuate a voltage applied to filaments to prevent the filament from vaporizing and destroying the lamp, and thereby extinguishing the initial arc before serious damage can occur. This will also detect an arc during normal operation of the lamp and generate the fault signal.
This solution is easy to implement while requiring no additional vehicle wiring. Additional vehicle wiring utilized in other solutions is expensive and ranges from $0.50 to over $1.00 per cut lead. Multiply this by millions of vehicles and alternative solutions cost manufacturers a lot of money. Warranty cost associated with replacing failed lamps is very expensive. This invention thus will lower the warranty replacement cost of multiple filament lamps. Depending on the lamp power driver design, the implementation of this invention can be both extremely easy to implement and very low in cost.